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i Y 398 Robert Rush Miller trout of Twin Lakes, Colorado, to the introduction of a deep-water competitor, the lake trout (Salvelinus namaycush)-a species that has raised havoc with native trouts in other lakes of the West. Chemical treatment.-The use of toxic chemicals, such as rote- none and toxaphene, for the control or eradication of fish populations may have serious consequences for the native species. Such a man- agement tool is being employed more and more widely in the control of "rough fish"; without prior determination of its harmful effects, this practice may needlessly exterminate localized species or relict populations (see above and Koster, 1957: 106). Its relatively indis- criminate use in streams has already reduced certain native fishes to dangerously low levels or has seemingly brought about local extinc- tion (Clark Hubbs, in litt., 1960). Conservationists should make a determined effort to prevent the decimation of aquatic biota in this way, if necessary through the enactment of protective legislation. the Changing climate.-This factor has been cited by sl initial cause of the recent excessive erosion in the arid Southwest (see on the contrary, Bailey, 1935, and Antev3 i ,1952) .Tree-ring and other records clearly indicate that periods of and and of wet conditions are recurrent and are a basic character of climate (Sears, 1957). However, the severe erosion of the past century is invariably associated with man's direct or indirect disturbance of the environment. Had the native vegetation been left alone the arroyo cutting that commenced in the 1880's would probably not have materialized. SUMMARY During the past 100 years, the aquatic habitats and the fish faunas of the American Southwest have undergone marked changes. Six or 7 species have become extinct and at least 13 others have been either locally exterminated or are so endangered that they too may Soon vanish. These 19 or 20 species constitute almost 20 per cent of the known aboriginal fishes of Western North America (north of Mexico). Reduced flow of surface waters has shrunk the ranges of many species and the distribution of others has been curtailed by the con- struction of barrier dams. Drastic reduction in ground-water levels has also eliminated or restricted the habitats of aquatic organisms. The establishment of some 36 species of exotic fishes has brought about replacement as well as reduction of the native forms through i L